1. Field of the Invention
The invention relates to switching amplifier systems, and more particularly to suppression of signal distortion of switching amplifier systems.
2. Description of the Related Art
A switching amplifier has higher power conversion efficiency than that of a class AB amplifier and therefore its use has become more and more popular. The switching amplifier, however, generates an output signal with rapid switching level and therefore induces significant electro magnetic interference (EMI). A switching amplifier converts an input signal into a ternary pulse width modulation (PWM) signal for driving an H-bridge of the switching amplifier. Referring to FIG. 1A, a schematic diagram of generation of a ternary PWM signal of a switching amplifier is shown. The ternary PWM signal has three output levels of +1, 0, and −1. When the amplitude of an input signal is greater than a threshold level, a PWM output signal with a +1 level and a pulse width in proportion to the amplitude of the input signal is generated. When the amplitude of the input signal is less than the threshold level, a PWM output signal with a −1 level and a pulse width in proportion to the amplitude of the input signal is generated.
When the switching amplifier converts an input signal into a PWM signal, signal distortion occurs and shaping errors are induced. Referring to FIG. 1B, a schematic diagram of shaping errors induced following generation of a PWM signal is shown. For example, an ideal waveform of a PWM signal is shown on an upper half of FIG. 1B. The ideal waveform comprises a pulse X1 in a first frame and a pulse X2 in a second frame. The pulse X1 has a level of +1, and the pulse X2 has a level of −1. When a switching amplifier performs a PWM conversion process, a real waveform is generated instead of the ideal waveform, as shown in the lower half of FIG. 1B. Due to a finite rising and falling speed of the real waveform, a shaping error E1 is induced to the real waveform of the pulse X1 when the value of the PWM signal changes from 0 to +1, and a shaping error E2 is induced to the real waveform of the pulse X1 when the value of the PWM signal changes from +1 to 0. Similarly, a shaping error E3 is induced to the real waveform of the pulse X2 when the value of the PWM signal changes from 0 to −1, and a shaping error E4 is induced to the real waveform of the pulse X2 when the value of the PWM signal changes from −1 to 0.
Thus, the area of the real waveform of a positive pulse X1 is equal to (X1−E1+E2), and the area of the real waveform of a negative pulse X2 is equal to (X2−E3+E4). When a PWM conversion process is performed, a total shaping error of (E2−E1) is therefore added to the positive pulse X1 of the PWM output signal, and a total shaping error of (E4−E3) is therefore added to the negative pulse X2 of the PWM output signal. A signal distortion is therefore generated on the PWM output signal according to whether the level of the input signal is positive or negative. Referring to FIG. 1C, a schematic diagram of a transfer curve of a PWM output signal is shown. A dotted line indicates a transfer curve between an input signal and an ideal PWM output signal. A solid line indicates a transfer curve between an input signal and a real PWM output signal. The difference between the solid line and the dotted line reflects the signal distortion based on a polarity of the input signal. The cross-over signal distortion generated in a PWM conversion process degrades performance of the switching amplifier. Thus, a method for avoiding the situation where signal distortion is induced by a PWM conversion process is therefore required to improve the performance of the switching amplifier.
Referring to FIG. 2, a block diagram of a closed-loop switching amplifier system 20 is shown. The switching amplifier system 20 comprises a digital signal processor 21, a noise shaper 22, a pulse width logic 23, an H-bridge 24 comprising power switches, and an analog-to-digital converter (ADC) 26. The digital signal processor 21 processes an input signal S21 to generate a signal S22. The noise shaper 22 performs a noise shaping process on the signal S22 according to a feedback signal S26 to generate a signal S23. The pulse width logic 23 converts the signal S23 from a PCM format to a PWM format to generate a signal S24. The H-bridge 24 amplifies the signal S24 to generate a signal S25 to drive the loudspeaker 25. The aforementioned cross-over signal distortion is induced by the power switches of the H-bridge 24. To suppress the signal distortion, the ADC 26 samples the signal S25 output by the power switch of the H-bridge 24 to obtain a signal S26, and the signal S26 is further fed back to the noise shaper as a feedback input. The switching amplifier system 20 therefore forms a closed loop and the signal distortion due to shape errors induced at the power switches of the H-bridge 24 is therefore suppressed. The ADC 26, however, occupies a large chip area and has a complex circuit structure. The hardware cost of the switching amplifier system 20 shown in FIG. 2 is therefore increased and the switching amplifier system is therefore rarely implemented.